db/simple: purge songs for unavailable decoder plugins on update

src/db/plugins/simple/Song.cxx

@@ -34,6 +34,14 @@ Song::Song(DetachedSong &&other, Directory &_parent) noexcept
 {
 }
 
+const char *
+Song::GetFilenameSuffix() const noexcept
+{
+	return target.empty()
+		? PathTraitsUTF8::GetFilenameSuffix(filename.c_str())
+		: PathTraitsUTF8::GetPathSuffix(target.c_str());
+}
+
 std::string
 Song::GetURI() const noexcept
 {

src/db/plugins/simple/Song.hxx

@@ -108,6 +108,16 @@ struct Song {
 
 	Song(DetachedSong &&other, Directory &_parent) noexcept;
 
+	gcc_pure
+	const char *GetFilenameSuffix() const noexcept;
+
+	/**
+	 * Checks whether the decoder plugin for this song is
+	 * available.
+	 */
+	gcc_pure
+	bool IsPluginAvailable() const noexcept;
+
 	/**
 	 * allocate a new song structure with a local file name and attempt to
 	 * load its metadata.  If all decoder plugin fail to read its meta